Conventionally, an amplifying device has been used in various kinds of electronic devices. For example, a radio communication device, such as a base station and a user terminal in a mobile communications system, uses an amplifying device that amplifies transmission power of a signal.
There has been known an amplifying device that uses, as illustrated in FIG. 1, a short stub that is a line having an electric length one fourth the wavelength (that is, λ/4) of a fundamental wave signal S1, as a bias circuit that supplies a certain bias voltage to an amplifying element 11. Hereinafter, the short tub is referred to as “λ/4 short stub” in some cases. FIG. 1 is a circuit diagram illustrating one example of the amplifying device in a first related art. The voltage output from a power source Vdd is supplied, as a bias voltage, to an amplifying element 11 via a λ/4 short stub TL1 that is a bias circuit. The fundamental wave signal S1 is amplified by the amplifying element 11, and a signal S2 after being amplified is output from the amplifying element 11. Furthermore, a bypass capacitor C1 is connected between the power source Vdd and a ground in parallel with the bias circuit. Hereinafter, components having the identical functions are given same numerals and their repeated explanations are omitted.
Furthermore, in order to improve amplification efficiency, “harmonic processing” that reflects, on the amplifying element, a harmonic signal out of the signals output from the amplifying element after being amplified may be performed. Hereinafter, the signals output from the amplifying element after being amplified are referred to as “amplified signals” in some cases. The amplification efficiency is, for example, specified by the formula “(output power of amplifying element)/(source power of amplifying element)”. When the amplifying element is a field effect transistor (FET), the amplification efficiency is referred to as “drain efficiency” in some cases. There has been known an amplifying device that uses, for example, an open stub that is a line having an electric length one eighth the wavelength (that is, λ/8) of the fundamental wave signal S1, as a reflection circuit that reflects a harmonic signal having a frequency twice the frequency of the fundamental wave signal (that is, “second harmonic signal”). Hereinafter, the open stub is referred to as “λ/8 open stub” in some cases. FIG. 2 is a circuit diagram illustrating one example of an amplifying device in a second related art. As illustrated in FIG. 2, a second harmonic signal S3 out of amplified signals output from the amplifying element 11 is reflected by a λ/8 open stub TL3 that is a reflection circuit, adjusted to a suitable reflection phase by a phase adjustment line TL2, and reaches the amplifying element 11. Because the second harmonic signal S3 is terminated by the λ/8 open stub TL3, an amplified signal S2′ at or after the connection point of the λ/8 open stub TL3 includes a small amount of second harmonic signal S3.
Examples of related-art are described in Japanese Laid-open Patent Publication No. 2011-035761.
Here, the amplifying device using the λ/4 short stub TL1 as a bias circuit (see FIG. 1) and the amplifying device using the λ/8 open stub TL3 as a reflection circuit (see FIG. 2) are combined in a simple manner to obtain an amplifying device illustrated in FIG. 3. FIG. 3 is a circuit diagram illustrating one example of an amplifying device in a third related art. However, in the amplifying device illustrated in FIG. 3, the λ/4 short stub TL1 as a bias circuit and the λ/8 open stub TL3 as a reflection circuit exist independently of each other and hence the arrangement area of the circuit become large.
In terms of the electric length of a line, the λ/4 short stub and the λ/8 open stub exhibit identical characteristics with respect to the second harmonic signal. Therefore, the λ/4 short stub can also serve as the λ/8 open stub. For this reason, as illustrated in FIG. 4, the λ/4 short stub TL1 is used as both the bias circuit of the amplifying element 11 and the reflection circuit with respect to a second harmonic signal, thus eliminating the λ/8 open stub TL3 (see FIG. 3) as a reflection circuit. Consequently, the arrangement area of the circuit in the amplifying device decreases, thus achieving the miniaturization of the amplifying device. FIG. 4 is a circuit diagram illustrating one example of an amplifying device in a fourth related art.
Here, the case in which the λ/4 short stub TL1 used as both the bias circuit and the reflection circuit is mounted on the amplifying device is considered.
First, a circuit model when the line in an amplifying device is an ideal line is illustrated in FIG. 5, and the simulation results of the frequency characteristics of the circuit model illustrated in FIG. 5 are illustrated in FIG. 6. FIG. 5 is a diagram illustrating one example of the circuit model using the ideal line, and FIG. 6 is a diagram illustrating the simulation results of the frequency characteristics of the circuit model using the ideal line. As illustrated in FIG. 5, the frequency of the fundamental wave signal is set to 2.14 GHz. Therefore, the frequency of the second harmonic signal is 4.28 GHz. The line that connects P1 and P2 in FIG. 5 corresponds to the line on the output side of the amplifying element 11 in FIG. 4, and each of the characteristic impedances of a line TLI and a line TLO is set to 50Ω in this case. One end of the λ/4 short stub TL1 whose characteristic impedance is 50Ω is connected to a point located between the line TLI and the line TLO. FIG. 6 illustrates the characteristics between P1 and P2 in FIG. 5. In FIG. 6, the continuous line indicates pass characteristics S21, and the dashed line indicates reflection characteristics S11.
When the line in the amplifying device is an ideal line in FIG. 6, a passing amount (the continuous line) is the maximum and a reflection amount (the dashed line) is the minimum at 2.14 GHz that is the frequency of the fundamental wave signal. On the other hand, a passing amount (the continuous line) is the minimum and a reflection amount (the dashed line) is the maximum at 4.28 GHz that is the frequency of the second harmonic signal. Therefore, as can be understood from FIG. 5 illustrating the circuit model using the ideal line, the λ/4 short stub TL1 is an ideal bias circuit with respect to the fundamental wave signal as well as being an ideal reflection circuit with respect to the second harmonic signal. That is, as can be understood from FIG. 5 illustrating the circuit model using the ideal line, the λ/4 short stub TL1 operates as an ideal element used as both the bias circuit and the reflection circuit.
Next, FIG. 7 illustrates a circuit model when the line in an amplifying device is a microstrip line, and FIG. 8 illustrates the simulation results of the frequency characteristics of the circuit model illustrated in FIG. 7. FIG. 7 is a diagram illustrating one example of a circuit model using the microstrip line, and FIG. 8 is a diagram illustrating the simulation results of the frequency characteristics of the circuit model using the microstrip line.
Here, a circuit actually formed by using the microstrip line is, as illustrated in FIG. 9 and FIG. 10, formed of a dielectric body 51 that constitutes a printed circuit board, a conductor pattern 52 formed on the surface of the dielectric body 51, and a ground (GND) plane 53 formed on a whole area of the back face of the dielectric body 51. FIG. 9 and FIG. 10 are diagrams each illustrating one example of the configuration of the circuit formed by using the microstrip line. FIG. 9 is an overall perspective view, and FIG. 10 is a cross-sectional view. The dielectric body 51 has a dielectric thickness D1. Each conductor pattern 52 has a pattern length L, a pattern width W, and a pattern thickness D2. The conductor pattern 52 is connected to the GND plane 53 via through holes 54 formed in the dielectric body 51.
In FIG. 7 and FIG. 8 as well as FIG. 5 and FIG. 6, the frequency of the fundamental wave signal is set to 2.14 GHz and the frequency of the second harmonic signal is set to 4.28 GHz. In order for a characteristic impedance to be 50Ω in the same manner as the case of FIG. 5, each of the pattern widths of lines MLI and MLO is set to 2.54 mm. Each of the pattern lengths of the lines MLI and MLO is set to 2 mm. Furthermore, in order for the characteristic impedance to be 50Ω in the same manner as the case of FIG. 5, the pattern width of a short stub ML1 is set to 2.54 mm identical with the pattern widths of the lines MLI and MLO. On the other hand, in order for an electric length to be λ/4, the pattern length of the short stub ML1 is set to 24.9 mm. Therefore, each of the pattern widths of connecting lines MT1, MT2, and MT3 located among the lines MLI, MLO, and the λ/4 short stub ML1 is set to 2.54 mm. Furthermore, the dielectric thickness of the dielectric body is set to 0.8 mm, and the relative dielectric constant of the dielectric body is set to 2.
A microstrip line A that connects P1 and P2 in FIG. 7 corresponds to the line on the output side of the amplifying element 11 in FIG. 4 in the same manner as the case of FIG. 5. Hereinafter, the microstrip line A is referred to as “main line A” in some cases. The λ/4 short stub ML1 is connected to the connecting line MT located between the line MLI and the line MLO. FIG. 8 illustrates the characteristics between P1 and P2 in FIG. 7. In FIG. 8, the continuous line indicates pass characteristics S21, and the dashed line indicates reflection characteristics S11.
As can be understood from FIG. 8, when the line in the amplifying device is a microstrip line that is generally used in an actual circuit, a passing amount (the continuous line) is the maximum, and a reflection amount (the dashed line) is the minimum at 2.14 GHz that is the frequency of the fundamental wave signal in the same manner as the case that the line in the amplifying device is an ideal line (see FIG. 6). On the other hand, the passing amount (the continuous line) is the minimum, and the reflection amount (the dashed line) is the maximum at 4.13 GHz, which is different from 4.28 GHz that is the frequency of the second harmonic signal. Therefore, the λ/4 short stub ML1 is an ideal bias circuit with respect to the fundamental wave signal. On the other hand, the λ/4 short stub ML1 is not an ideal reflection circuit with respect to the second harmonic signal. That is, in the circuit model using the microstrip line illustrated in FIG. 7, the reflection amount of the second harmonic signal in the λ/4 short stub used as both the bias circuit and the reflection circuit decreases, thus lowering amplification efficiency compared with the case of the circuit model using the ideal line (see FIG. 5).